Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves and clothes dryers. Ceramic igniter production requires constructing an electrical circuit through a ceramic component, a portion of which is highly resistive and rises in temperature when electrified by a wire lead. One conventional igniter, the Mini-Igniter.TM., available from the Norton Company of Milford, N.H., is designed for 8 volt though 48 volt applications and has a composition comprising aluminum nitride ("AlN"), molybdenum disilicide ("MoSi.sub.2 "), and silicon carbide ("SIC"). As the attractiveness of the Mini-Igniter.TM. has grown, so has the number of applications requiring small igniters with rated voltages exceeding the conventional 24 volts. However, when used in such applications, the 24 V Mini-Igniter.TM. is subject to temperature runaway and so requires a transformer in the control system to step down from conventional line voltage (i.e., 120 volts). Accordingly, there is a need for small, higher voltage igniters designed for either 120 or 230 line voltage applications which do not require an expensive transformer but still possess the following requirements set by the appliance and heating industries to anticipate variation in line voltage:
______________________________________ Time to design temperature &lt;5 sec Minimum temperature at 85% of design voltage 1100.degree. C. Design temperature at 100% of design voltage 1350.degree. C. Maximum temperature at 110% of design voltage 1500.degree. C. Hot-zone Length &lt;1.2-1.5" Power 65-100 W. ______________________________________
Because the amperage used for these high voltage applications will likely be comparable to that used in 24 volt applications (i.e., about 1.0 amp), the increased voltage will likely be realized by increasing the resistance of the igniter.
The resistance of any body is generally governed by the equation EQU Rs=Ry.times.L/A,
wherein
Rs=Resistance; PA1 Ry=Resistivity; PA1 L=the length of the conductor; and PA1 A=the cross-sectional area of the conductor. PA1 (a) between 5 and 50 v/o MoSi.sub.2, and PA1 (b) between 50 and 95 v/o of a material selected from the PA1 (a) between about 50 and about 80 v/o of an electrically insulating ceramic having a resistivity of at least about 10.sup.10 ohm-cm; PA1 (b) between about 10 and about 45 v/o of a semiconductive material having a resistivity of between about 1 and about 10.sup.8 ohm-cm; PA1 (c) between about 5 and about 25 v/o of a metallic conductor having a resistivity of less than about 10.sup.-2 ohm-cm; and PA1 (d) between about 0.5 and about 20 v/o of a resistivity-enhancing compound selected from the group consisting of metallic oxides, metallic oxynitrides, rare earth oxides, rare earth oxynitrides, and mixtures thereof
Because the single leg length of conventional 12 V and 24 V igniters is already about 1.2 inches, it can not be increased significantly without reducing its commercial attractiveness. Similarly, the cross-sectional area of the smaller igniter, between about 0.0010 and 0.0025 square inches, will probably not be decreased for manufacturing reasons. Therefore, it appears that the desired increase in the resistance of the small, high voltage igniters will be realized by increasing its resistivity.
Because the Mini-Igniter.TM. is comprised of one highly resistive material (AlN), one moderately resistive material (SiC), and one highly conductive material (MoSi.sub.2), one obvious avenue for increasing the igniter's resistivity is to reduce its MoSi.sub.2 and SiC contents while adding AlN. However, one trial composition (containing about 76 volume percent ("v/o" or "vol %") AlN, 9 v/o MoSi.sub.2, and 15 v/o SiC) was found to be unsatisfactory in that it not only was slow to reach the design temperature (due to low MoSi.sub.2 levels), it also possessed a significant negative temperature coefficient of resistivity ("NTCR") and so was subject to temperature runaway above about only 1350.degree. C. A NTCR means that as the temperature of the igniter increases, its resistance decreases. This decrease makes the igniter hotter than it would be if the resistance was constant. If the NTCR is too extreme, the igniter is slow and cool at 85% and unstable at 110% of the rated voltage. Indeed, such an igniter may exhibit runaway at less than the 110% rating, in which case the amperage and temperature continue to rise even at a constant voltage until failure (burnout) occurs. Rather, it is preferable for the igniters to possess a positive temperature coefficient of resistance ("PTCR") or a moderate NTCR. Whereas a ceramic having a PTCR increases in resistivity when its temperature is increased from 1000.degree. C. to 1400.degree. C., a ceramic having a moderate NTCR decreases in resistivity by less than 25% when its temperature is increased from 1000.degree. C. to 1400.degree. C. Either a PTCR or a moderate NTCR would allow for a more gradual temperature increase with increasing voltage, which is critical for 120 V applications because, as explained above, the igniter must operate stably over a broad range of voltage.
U.S. Pat. No. 5,405,237 ("the Washburn patent") discloses compositions suitable for the hot zone of a ceramic igniter comprising:
group consisting of silicon carbide, silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium aluminate, silicon aluminum oxynitride, and mixtures thereof. However, each example disclosed in the Washburn patent (and companion U.S. Pat. No. 5,085,804) uses only a) AlN or Si.sub.3 N.sub.4, b) MoSi.sub.2 and c) SiC (with some examples also adding MgCO.sub.3). As discussed above, it is believed these systems are not readily conducive to producing commercially viable ceramic igniters which are stable at high voltages. Although the Washburn patent does disclose a 220 V igniter made from 50 v/o AlN, 42.2 v/o SiC and 7.8 v/o MoSi.sub.2, the low MoSi.sub.2 level in this igniter dramatically constrains the speed with which this igniter reaches its design temperature.
Accordingly, it is the object of the present invention to find a highly resistive mini-igniter composition which does not experience temperature runaway at high temperatures and meets the above-discussed time and temperature constraints of high voltage applications.